public class Pailiezuhe {
  public static void main(String args[]) {
     int [] temp = new int [args.length];
     int sum;
     for(int i=0;i<args.length;i++) {
        temp[i]=Integer.parseInt(args[i]);
     }
     sum = fact(temp[0],temp[1]);
     if(sum == 0) System.out.println("error");
     else System.out.println(sum);
  }
  public static int fact(int n,int m) {
       if(m==1) return n;
       else  if( m==0 ||  m==n) return 1;
       else  if(n<m || n==0) return 0;
       else  return fact(n-1,m-1)+fact(n-1,m);
  }
}
